1. Field of the Invention
The present invention relates to image processing for reproducing a color and texture.
2. Description of the Related Art
Color materials for inkjet printers include dye-based ink and pigment-based ink. The dye-based ink contains water as the main component. Such a color material dissolved in a solvent easily penetrates into a recording medium. Accordingly, the dye-based ink offers excellent color reproduction and the surface of the recording medium subjected to recording using the dye-based ink is even. The dye-based ink is therefore excellently suitable for photo printing. Disadvantageously, if the recording medium subjected to recording using the dye-based ink is exposed to light or air, a recorded image fades, namely, color fading occurs over time because dye molecules are easily decomposed.
On the other hand, the pigment-based ink sharply expresses the outline of a character or the like and contrast and has excellent light-fastness and water-fastness. However, the pigment-based ink hardly penetrates into a recording medium because particles of the pigment-based ink having a size ranging from tens of nanometers to several micrometers are dispersed in a solvent. Accordingly, the particles deposit on the surface of the recording medium, so that the surface of a recording area in a recorded image slightly differs in fine structure from that of a non-recording area. Disadvantageously, the smoothness of the entire surface of the recording medium is lost. Such a phenomenon that glossiness varies depending on color density or color in a recorded image, namely, gloss unevenness occurs. If rough areas and matte areas are mixed in an image due to gloss unevenness, these areas are likely to be recognized as image defects in, especially, photograph printing. Japanese Patent Laid-Open No. 2002-307755 discloses a method of applying clear ink in an image area with little colored ink to reduce unevenness on the surface of an image and prevent diffused reflection of light on the surface, thus maintaining gloss evenly.
Gloss unevenness is not confined to inkjet printers using pigment-based ink. Such a problem also occurs in electrophotographic printers for fixing toner onto a recording medium to record an image. U.S. Patent Laid-Open No. 2002-0167681 discloses a method of maintaining gloss evenly using clear toner in a manner similar to the above-described inkjet printer using the pigment-based ink.
Image reproduction using a printer was initially intended to faithfully reproduce colors of an object. After that, demands for faithful reproduction of gloss and physical characteristics of the surface of the object in addition to the color reproduction have grown. In this specification, the physical characteristics other than the grayscale levels of colors of an object will be called “texture” and reproducing texture will be called “texture reproduction” hereinafter. The texture of an object includes not only gloss but also the structure of the surface of the object and the bidirectional reflectance distribution function thereof. The bidirectional reflectance distribution function is a function that defines how light is reflected in each direction when light coming from a certain direction is incident on a certain point on the surface of an object.
To meet the demands for texture reproduction, a clear color material is used as a material that adds new texture, which has not been provided in conventional printing, to a printed material. For example, clear ink is applied to a character area to make the glossiness of the character area differ from that of a portion surrounding the character area, thus decorating a character.
Further advanced techniques include a technique of reproducing glossinesses of an object in a printed material as faithful as possible. This technique will be called “gloss reproduction” hereinafter. According to a method disclosed in U.S. Pat. No. 6,733,105, in order to reproduce glossinesses of an object, illumination light is applied to the object to measure gloss signals and a formation pattern of 16 gloss control materials according to the levels of the gloss signals is used. The gloss control materials are applied in accordance with the formation pattern to control gloss, thus reproducing the glossinesses of the object.
The above-described methods disclosed in Japanese Patent Laid-Open No. 2002-307755 and U.S. Patent Laid-Open No. 2002-0167681 intend to entirely ensure even glossiness of a printed material without taking into glossinesses of an object. In other words, the methods disclosed in Japanese Patent Laid-Open No. 2002-307755 and U.S. Patent Laid-Open No. 2002-0167681 do not perform gloss reproduction. The use of a formation pattern for specifying how to apply gloss control materials is disclosed by R. Floyd and L. Steinberg in “An Adaptive Algorithm for Spatial Greyscale”, Proceeding of the S.I.D., Vol. 17/2, 1976, pp. 75-76 (hereinafter, referred to as “Non-patent Document 1”). However, the formation pattern can express glossinesses corresponding to the limited number of grayscale levels. In addition, if a periodic pattern is generated by a formation pattern, the periodic pattern is recognized as image quality degradation as in a dither pattern obtained by quantizing the grayscale levels of a color. Furthermore, it is difficult to perform fine adjustment on glossiness because the texture of a recording medium or that of a color material is not taken into consideration.